Where is the verse om purnam adah .. from – why is it included as the Ishopanishad invocation?

Transcript

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Question.
Where does the invocation of the Isopanishad come from originally? And if it is not a part of the Isopanishad, why is it included as the invocation? Answer. The text Om Purnamadah Purnamidam Purnat Purnamudachyate Purnasya Purnamadaya Purnamevashishyate is the first, is from the Bdhyadharanikupshud, Bdhyad 5.1.1 and there it is used also as an invocation. So, the previous chapters have had various contemplations, analysis of the nature of reality pointing towards the supreme reality of Brahman and there are further contemplations on Brahman in this 5th chapter, in this 5th chapter of the Bdhyadharanikupanishad and that discussion is launched with this invocatory verse which gives us the overarching principle in approaching the approaching the discussion about Brahman, that Brahman is inexhaustible that even if innumerable emanations come from the Brahman still Brahman remains inexhaustible.
So, the Bdhyadharanikupanishad is one of the most important Upanishads in the broad Vedic canon and it is extensively quoted even by Racharyas, especially when the discussion about Upanishads and how the Upanishadic body of knowledge also points to the Bhakti conclusions for that purpose. Quoting from the Bdhyadharanikupanishad is considered very important. So, Isopanishad is the short as well as very powerful, very profound Upanishad that Shri Prabhupada has commented upon and to his whole purpose in using the Isopanishad, in commenting on the Isopanishad is to show how the absolute truth is personal and yet the absolute truth is not personal simply like us.
We are limited persons. So, the unlimitedness of the absolute needs to be kept in mind while also understanding the personal attribute of the absolute. So, of course, that inconceivable and nature of the absolute will be talked about repeatedly in the Isopanishad itself but to stress the point that the absolute is universe, universe is inexhaustible, that the absolute is the source of everything and still remains not exhausted.
To stress that basic principle of the infinitude of Brahman and then to develop on it to point out to how within this infinitude also, the absolute truth can be personal. So, Prabhupada uses this standard which is often frequently quoted. In fact, it is one of the more well-known verses of the Brihadaranyaka Upanishad and because it gives one of the essential principles of Upanishadic wisdom, because it serves, it fits into the flow of Isopanishad, especially as Sri Prabhupada is analyzing and presenting it and because as an invocatory verse, its use has already become extended beyond being simply seen as a verse in a particular Upanishad to being seen as a frequently chanted mantra or verse or invocatory prayer.
So, just as Tamsoma, Jyotirgama also comes from the Brihadaranyaka Upanishad but it is quoted widely far outside the context also. So, similarly the invocation Om Podam Adah Podam It is widely known beyond the Vedic context, beyond the context of that particular text and so Sri Prabhupada uses it to give us a starting point for our exploration of the Isopanishad.